1. Field of the Invention
The present invention relates to a connection structure for connecting an optical fiber bundle to a single optical fiber, and also to a multi-core ferrule for holding the optical fiber bundle.
2. Description Related to the Prior Art
Since optical fibers are capable of transmitting a large volume of information, they are widely used as optical waveguides in the communication field and the industrial field. The optical fibers are formed of, for example, a core having a diameter of 205 μm and a clad surrounding the core and having an outer diameter of 250 μm. The outside of the clad is usually protected with a coating material.
When the optical fibers are connected, a ferrule is attached to an end of each optical fiber so that their axes are aligned parallel to each other. The ferrule has a cylindrical shape and has a through hole extending in its axial direction. The optical fiber is inserted into this through hole and fixed thereto using an adhesive material such that an end surface of the optical fiber approximately coincides with an end surface of the ferrule.
The ferrule is loaded inside a plug. Two plugs are inserted from opposite ends into an adaptor. The adaptor has a sleeve, and the ferrules are inserted from opposite ends into the sleeve, thereby the end surfaces of the ferrules are pressed against each other. These two optical fibers are aligned inside the sleeve and optically connected.
Optical fiber connection structures are broadly classified into three types: a single/single-core connection structure, a multi/multi-core connection structure, and a multi/single-core connection structure. In the single/single-core connection structure, a single optical fiber is connected to another single optical fiber (see for example, JIS C5973, F04 type single-core optical fiber connector). In the multi/multi-core connection structure, an optical fiber bundle which is formed of a plurality of optical fibers is connected to another optical fiber bundle (see for example, JIS C5982, F13 type multi-core optical fiber connector). In the multi/single-core connection structure, an optical fiber bundle which is formed of a plurality of optical fibers (see for example, Japanese Patent Laid-Open Publication No. 55-033130 and Japanese Patent Laid-Open Publication No. 03-044603) is connected to a single optical fiber.
By the way, if there is a gap between the end surfaces of the optical fibers, Fresnel reflection occurs at the interface between the optical fiber and the air in the gap, which results in optical loss. The Fresnel reflection is generally prevented by a physical contact (PC) connection for physically contacting the optical fibers closely, and thereby minimizing the optical loss.
In the single/single-core connection structure, as shown in page 6 and FIG. 5 of Japanese Patent Laid-Open Publication No. 10-260336 for example, a ferrule holding an optical fiber is finished to have a convex spherical end surface with a curvature radius of 15 mm to 25 mm, and two ferrules, each holding the optical fiber, are pressed against each other, thereby elastically deforming their end surfaces. Owing to this, there is no gap formed between the end surfaces, thus ensuring the PC connection. In Japanese Patent Laid-Open Publication No. 2002-333528, optical fibers are protruded from end surfaces of ferrules, and thereby ensuring the PC connection. Japanese Patent Laid-Open Publication No. 2000-180664 discloses the single/single-core connection structure which does not hire the PC connection. In this publication, ferrules are finished to have flat end surfaces, and the flat end surfaces are faced against each other. In addition, Japanese Patent Laid-Open Publication No. 2004-219567 discloses conditions (conditional equations for a curvature radius of an end surface of a ferrule, for the number of optical fibers in a bundle, etc.) for assuring the PC connection in the multi/multi-core connection structure.
In the multi/single-core connection structure, the PC connection is assured by finishing an end surface of each ferrule into a convex spherical surface (hereinafter referred to as convex surface), and pressing the ferrules, each holding a single optical fiber or an optical fiber bundle, against each other to elastically deform their end surfaces in the same manner as the single/single-core connection structure. According to the single/single-core connection structure disclosed in the Japanese Patent Laid-Open Publication No. 2002-333528, however, it is difficult to uniformly protrude respective optical fibers composing the optical fiber bundle, and therefore the PC connection cannot be assured.
According to the single/single-core connection structure disclosed in the Japanese Patent Laid-Open Publication No. 2000-180664, the ferrules have flat end surfaces and the sleeve needs to be formed with high precision, which results in a rise in production cost for assuring the connection.
Note that the conditional equations disclosed in the Japanese Patent Laid-Open Publication No. 2004-219567 are applied only for connecting the single optical fiber to another single optical fiber of the optical fiber bundles, and therefore not applicable in the multi/single-core connection structure.